This invention relates to a cathode ray tube operational circuit means and more particularly to an improvement in the circuit means for energizing a heater element associated with thermionic emission means within a cathode ray tube.
In a color display apparatus, such as a television receiver, the operational mode of the cathode ray tube (CRT) is achieved when the temperature of the thermionic emissive cathode material therein is raised to a thermal level to promote the generation of a stream of electrons, the resultant beam being thence controlled to effect the full operational mode of the tube. The required temperature level of the thermionic cathode emission means is conventionally effected by an electric heating element proximally associated therewith. In television and allied dynamic display apparatus it has been the desire to effect operation of the equipment as soon as possible after activation of the power turn-on switching means. For example, television receivers employing solid state circuitry experience a time lag existent between the activation of power switching and the resultant reproduction of correlated video and audio responses, such being dependent primarily upon the time span required by the heater means in the CRT to bring the temperature of the cathode or cathodes therein to the thermal level required for the initiation of operational electron emission. To reduce the span of time between the initiatory power switching and the subsequent visual response of the display apparatus, a practice developed in the art whereby a stand-by potential was applied to the heater or heaters in the cathode ray tube to maintain the cathode temperature at a predetermined thermal level. In such an arrangement, switching on of the apparatus supplied the additional heater voltage required to raise the cathode temperature to full electron emission level, thereby quickly inaugurating operational response of the apparatus. While application of the stand-by potential to the heater circuitry aptly fulfilled the desire for rapid initial operation, it, in general, constituted an unproductive utilization of electrical energy.
In striving to achieve more efficient usage of electrical energy in a television receiver, there was developed in the prior art, a rapid warm-up type of heater circuit which included a series coupled temperature responsive resistive element. Upon activation of this circuit, the room-ambient resistance of the element allowed the passage of a surge of current which then decreased to the normal heater operating level as the temperature of the element increased; the resultant being a predetermined resistive load in the circuit. While the rapid warm-up of the heater is accomplished by this circuit and efficiency of energy usage improved thereby, inclusion of the temperature responsive resistive element therein constitutes heat-related energy consumption which is a deterrent to the realization of still greater efficiency in electrical energy utilization.